Hi Eric, On Mon, Sep 30, 2024 at 03:05:38PM +0200, Eric Auger wrote: > Hi Mostafa, > > On 9/27/24 18:17, Mostafa Saleh wrote: > > Hi All, > > > > Background > > ========== > > I have been looking into assigning simple devices which are not DMA > > capable to VMs on Android using VFIO platform. > > > > I have been mainly looking with respect to Protected KVM (pKVM), which > > would need some extra modifications mostly to KVM-VFIO, that is quite > > early under prototyping at the moment, which have core pending pKVM > > dependencies upstream as guest memfd[1] and IOMMUs support[2]. > > > > However, this problem is not pKVM(or KVM) specific, and about the > > design of VFIO. > > > > [1] https://lore.kernel.org/kvm/20240801090117.3841080-1-tabba@xxxxxxxxxx/ > > [2] https://lore.kernel.org/kvmarm/20230201125328.2186498-1-jean-philippe@xxxxxxxxxx/ > > > > Problem > > ======= > > At the moment, VFIO platform will deny a device from probing (through > > vfio_group_find_or_alloc()), if it’s not part of an IOMMU group, > > unless (CONFIG_VFIO_NOIOMMU is configured) > > > > As far as I understand the current solutions to pass through platform > > devices that are not DMA capable are: > > - Use VFIO platform + (CONFIG_VFIO_NOIOMMU): The problem with that, it > > taints the kernel and this doesn’t actually fit the device description > > as the device doesn’t only have an IOMMU, but it’s not DMA capable at > > all, so the kernel should be safe with assigning the device without > > DMA isolation. > > > > - Use VFIO mdev with an emulated IOMMU, this seems it could work. But > > many of the code would be duplicate with the VFIO platform code as the > > device is a platform device. > > > > - Use UIO: Can map MMIO to userspace which seems to be focused for > > userspace drivers rather than VM passthrough and I can’t find its > > support in Qemu. > In case you did not have this reference, you may have a look at Alex' > reply in > https://patchew.org/QEMU/1518189456-2873-1-git-send-email-geert+renesas@xxxxxxxxx/1518189456-2873-5-git-send-email-geert+renesas@xxxxxxxxx/ > > > > One other benefit from supporting this in VFIO platform, that we can > > use the existing UAPI for platform devices (and support in VMMs) > > > > Proposal > > ======== > > Extend VFIO platform to allow assigning devices without an IOMMU, this > > can be possibly done by > > - Checking device capability from the platform bus (would be something > > ACPI/OF specific similar to how it configures DMA from > > platform_dma_configure(), we can add a new function something like > > platfrom_dma_capable()) > > > > - Using emulated IOMMU for such devices > > (vfio_register_emulated_iommu_dev()), instead of having intrusive > > changes about IOMMUs existence. > > > > If that makes sense I can work on RFC(I don’t have any code at the moment) > So if I understand correctly, assuming you are able to safely detect the > device is not DMA capable you would use the > > vfio_register_emulated_iommu_dev() trick. Is that correct? Yes, my guess the challenge here would be discovering that through the platform bus. Thanks, Mostafa > > Thanks > > Eric > > > > > Thanks, > > Mostafa > > >
